In current practice PMA STT MRAMs, magnetizations of both the MTJ Free Layer (FL) and the Reference Layer (RL) are perpendicular to the plane of MTJ layers and switch between low-resistivity parallel (Rp) and high-resistivity antiparallel (Rap) states which represent logical “0” and “1,” respectively. To effect proper writing operations, the PMA of the FL should be high enough to provide thermal stability for data retention while also being low enough to permit STT switching, and the PMA of the RL should be higher than that of FL—high enough to keep the magnetization of RL fixed and protected from STT disturbance or switching.
For exercising a proper reading operation, the resistance difference between the two memory states, expressed as the MTJ Magneto-resistance Ratio (MR=(Rap−Rp)/Rp), should be high enough to provide sufficient readback signal. It is therefore important that MTJ elements combine strong well-controlled PMA with high MR.
The MTJ elements disclosed to date have utilized a (100) oriented MgO layer as the tunnel barrier, since (100) is the orientation that MgO naturally assumes during deposition/oxidation. This MgO layer is then in contact, at both top and bottom, with ferromagnetic layers that, during the subsequent annealing step, will adopt the crystalline orientation of the MgO layer.
A commonly used configuration is [amorphous CoFeB]/MgO/[amorphous CoFeB]. Upon annealing, crystallization progresses from the MgO interfaces, transforming the structure to crystalline [bcc (100) CoFeB]/(100) MgO/[bcc (100) CoFeB] which is then used to effect the MTJ MR device.
Although such a system may possess some PMA due to interfacial anisotropy between MgO and CoFeB, it is necessary to enhance such PMA by adding some bulk PMA materials and/or multilayers. However, the best choices for enhancing the PMA (such as [Pt/Co(Fe)], [Pd/Co(Fe)], [Ni/Co] magnetic layers) “prefer” a fcc (111) crystal orientation which, unfortunately, is incompatible with the bcc (100) orientation induced by the MgO.
As a result, although their detailed materials selections may vary, state of the art MTJ elements are variations of the following two general structures (in which the FL can be either above or below the MgO layer) which are illustrated in FIGS. 1a and 1b:     Shown in FIG. 1a is: Bottom layer(s) 10a/[fcc (111) PMA layer 11a]/[bcc (100) MR layer 13a]/(100) MgO/[bcc (100) MR layer 14a]/[fcc (111) PMA layer 16a]/Cap layer 17. Note that an optional transitional layer between layers 14a and 16a is not shown.    Shown in FIG. 1b is: Bottom layer(s) 10b/[fcc (111) PMA layer 11b]/[bcc (100) MR layer 13b]/(110) MgO/[bcc (111) MR layer 14b]/[fcc (111) PMA layer 16b]/Cap layer 17. Note that an optional transitional layer between layers 14b and 16b is not shown.
In that, in these structures as a matter of design choice, the FL can be either above or below the MgO layer, the FL and RL can be either simple or Synthetic Antiferromagnetic (SAF) layers, and an additional magnetic Dipole Layer (DL) could be added above or below that stack to improve the symmetry of FL switching.
Some of the layers in FIGS. 1a or 1b may be present, or not, and some materials selections may be different (e.g., L10 ordered phase PMA material instead of fcc (111) PMA), but one important feature remains common:
In at least in one place in the structure, below or above the MgO or both, there is a transition between two different and incompatible crystalline orientations. This mismatch leads to several drawbacks in current practice MTJs as follows:
(a) if the “fcc (111) PMA layer” to “bcc (100) MR layer” transition is part of the bottom layers (i.e. below the MgO as shown in FIG. 1a), neither the (111) nor (100) textures can fully develop at their interface, and the PMA of the PMA layer and the MR of the MR layer are both weakened;
(b) introduction of a transitional layer to mitigate drawback (a) is of only limited help, since the transitional layer decouples the PMA layer from the MR layer making it more difficult for the MR layer to remain in PMA mode; and (c) if the “fcc (111) PMA layer” to “bcc (100) MR layer” transition appears in the top layers (i.e., above the MgO as also shown in FIG. 1a), the situation is even worse due to lack of proper seeding for the “fcc (111) PMA layer” (the role played by “underlayer” in the bottom layers' case) makes it very difficult to grow the PMA layer on top of the MR layer. This is in addition to drawbacks (a) and (b) above, which still apply.
All these drawbacks are absent from the device that will be disclosed below.